Semiconductor structures including square cuts in single crystal silicon and method of forming same

ABSTRACT

A single crystal silicon etching method includes providing a single crystal silicon substrate having at least one trench therein. The substrate is exposed to a buffered fluoride etch solution which undercuts the silicon to provide lateral shelves when patterned in the &lt;100&gt; direction. The resulting structure includes an undercut feature when patterned in the &lt;100&gt; direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/565,557, filed Sep. 23, 2009, now U.S. Pat. No. 7,973,388, issuedJul. 5, 2011, which is a divisional of application Ser. No. 11/445,718,filed on Jun. 2, 2006, now U.S. Pat. No. 7,628,932, issued Dec. 8, 2009,each assigned to the Assignee of the present application. Thisapplication is also related to U.S. patent application Ser. No.11/445,911, filed Jun. 2, 2006, now U.S. Pat. No. 7,625,776, issued Dec.1, 2009, and U.S. patent application Ser. No. 11/445,544, filed Jun. 2,2006, now U.S. Pat. No. 7,709,341, issued May 4, 2010, each assigned tothe Assignee of the present application. The disclosure of each of thepreviously referenced documents is hereby incorporated by reference inits entirety.

TECHNICAL FIELD

The present invention relates generally to methods for undercuttingsingle crystal silicon using wet etchants. More particularly, thepresent invention relates to methods for creating square undercuts insingle crystal silicon and resulting structures.

BACKGROUND

Higher performance, lower cost, increased miniaturization ofsemiconductor components, and greater packaging density of integratedcircuits are ongoing goals of the computer industry. One way to reducethe overall cost of a semiconductor component is to reduce themanufacturing cost of that component. Lower manufacturing costs can beachieved through faster production as well as in reduction in the amountof materials used in fabricating the semiconductor component. In recentyears, the semiconductor industry has greatly expanded its emphasis indevelopment and production of electro-optical components, such as, forexample, charge-coupled devices (CCDs) and, more recently, CMOS imagers.As with other semiconductor components, there is a continued drivetoward higher performance parameters and greater yields at ever-lowercosts.

Micro-electromechanical systems (“MEMS”) is another technology receivinga great deal of attention in many industries, including the electronicsindustry. MEMS integrate microminiature electrical and mechanicalcomponents on the same substrate, for example, a silicon substrate,using microfabrication technologies to form extremely small apparatuses.The electrical components may be fabricated using integrated circuitfabrication (“IC”) processes, while the mechanical components may befabricated using micromachining processes that are compatible with theintegrated circuit fabrication processes. This combination of approachesmakes it possible, in many instances, to fabricate an entiremicrominiature system on a chip using conventional manufacturingprocesses. However, there remain many shortcomings in existingfabrication technologies that limit the types and sizes of MEMScomponents and assemblies, which may be fabricated.

Conventional IC processing for DRAM, microprocessors, etc., arecurrently performed on (100) silicon. Potassium hydroxide and TMAH maybe used to create vertical etches in (110) silicon by using (110)substrate wafers or causing the recrystallization of the surface of asubstrate wafer to have a (110) crystal orientation. However, theresultant structures are not always desirable and may introduce costly,additional processing steps and procedures to the fabrication processand create a low performance device.

Various conventional chemistries have been used to etch silicon. Forexample, both single crystal and polycrystalline silicon are typicallywet etched in mixtures of nitric acid (HNO₃) and hydrofluoric acid (HF).With use of such etchants, the etching is generally isotropic. Thereaction is initiated by the HNO₃, which forms a layer of silicondioxide on the silicon, and the HF dissolves the silicon oxide away. Insome cases, water is used to dilute the etchant, with acetic acid(CH₃COOH) being a preferred buffering agent.

In some applications, it is useful to etch silicon more rapidly alongone or more crystal planes relative to others. For example, in thediamond lattice of silicon, generally the (111) plane is more denselypacked than the (100) plane, and thus the etch rates of (111) orientatedsurfaces are expected to be lower than those with (100) orientations.Bonding orientation of the different planes also contributes to etchantselectivity to exposed planes. One etchant that exhibits suchorientation-dependent etching properties consists of a mixture of KOHand isopropyl alcohol. For example, such a mixture may etch about onehundred (100) times faster along (100) planes than along (111) planes.

Hydroxide etchants and TMAH may be used to create a vertical undercut in(100) silicon. FIGS. 1A-2B show a silicon etch performed with differentetchant solutions in both the standard silicon orientation (FIG. 1A andFIG. 2A) and 45° rotation (FIG. 1B and FIG. 2B). In the standardorientation, a mask is aligned along the <110> directions. The {111}planes define the sidewalls which are sloped from (100) surface plane.With the 45° rotation, the mask is aligned along the <100> direction. InFIG. 1, the etchant was dilute NH₄OH applied at 26° C. and in FIGS. 2Aand 2B, the etchant was dilute TMAH applied at 26° C. While the twoetchants display different selectivity, both undercut the silicon 10 andcreate beveled edges or chamfers 12. The beveled edges may beundesirable for some applications and may limit the spacing ofcomponents on the integrated circuit.

Accordingly, it would be desirable to create square undercuts in (100)silicon without beveled edges, or chamfers and/or to manipulate theshape of the undercut. Further, it would be desirable to create alateral shelf in (100) silicon using wet etch chemistry.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIG. 1A is a cross-sectional view of single crystal silicon masked alongthe <110> direction and undercut with NH₄OH applied at 26° C. FIG. 1Bshows of single crystal silicon masked along the <100> direction andundercut with NH₄OH applied at 26° C.

FIG. 2A is a cross-sectional view of single crystal silicon masked alongthe <110> direction and undercut with dilute TMAH applied at 26° C. FIG.2B shows single crystal silicon masked along the <100> direction andundercut with dilute TMAH applied at 26° C.

FIG. 3A is a cross-sectional view of single crystal silicon masked alongthe <110> direction and undercut with a buffered fluoride etch solutionof the present invention applied at 23° C. FIG. 3B shows single crystalsilicon masked along the <100> direction and undercut with a bufferedfluoride etch solution of the present invention applied at 23° C.

FIGS. 4A-11D show a single crystal silicon wafer at various stages in afabrication process according to one embodiment of the presentinvention. FIG. 4A is a plan view of single crystal silicon waferaccording to an embodiment of the present invention. FIG. 4B is across-sectional view of the same single crystal silicon wafer takenalong line -4B- of FIG. 4A.

FIG. 5A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 5B is a cross-sectional viewof the same single crystal silicon wafer taken along line -5B- of FIG.5A.

FIG. 6A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 6B is a cross-sectional viewof the same single crystal silicon wafer taken along line -6B- of FIG.6A.

FIG. 7A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 7B is a cross-sectional viewof the same single crystal silicon wafer taken along line -7B- of FIG.7A.

FIG. 8A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 8B is a cross-sectional viewof the same single crystal silicon wafer taken along line -8B- of FIG.8A.

FIG. 9A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 9B is a cross-sectional viewof the same single crystal silicon wafer taken along line -9B- of FIG.9A.

FIG. 10A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 10B is a cross-sectional viewof the same single crystal silicon wafer taken along line -10B- of FIG.10A. FIG. 10C is a cross-sectional view of the single crystal siliconwafer of FIG. 10A taken along line -10C- of FIG. 10A.

FIG. 11A is a plan view of single crystal silicon wafer according to anembodiment of the present invention. FIG. 11B is a cross-sectional viewof the same single crystal silicon wafer taken along line -11B- of FIG.11A. FIG. 11C is a cross-sectional view of the single crystal siliconwafer of FIG. 11A taken along line -11C- of FIG. 11A. FIG. 11D is across-sectional view of the single crystal silicon wafer of FIG. 11Ataken along line -11D- of FIG. 11A.

FIGS. 12A-12E show a progressive undercut etch of single crystal siliconusing a buffered fluoride etch solution of the present invention. Thetrenches are in the <100> direction on (100) silicon.

FIGS. 13A-13D show a progressive undercut etch of single crystal siliconusing a buffered fluoride etch solution of the present invention afterexposure to NH₄OH. The trenches are in the <100> direction on (100)silicon.

FIGS. 14A and 14B show transmission electron micrographs (TEMs) of anintegrated PSOI DRAM access structure.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof; and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

The terms “wafer” and “substrate” as used in the following descriptioninclude any structure having an exposed surface with which to form theintegrated circuit (IC) structure of the invention. The term substrateis understood to include semiconductor wafers. The term substrate isalso used to refer to semiconductor structures during processing, andmay include other layers that have been fabricated thereupon. Both waferand substrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator, aswell as other semiconductor structures well known to those of ordinaryskill in the art. The term “conductor” is understood to includesemiconductors, and the term “insulator” is defined to include anymaterial that is less electrically conductive than the materialsreferred to as conductors. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims, along with thefull scope of equivalents to which such claims are entitled.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizontal asdefined above. Prepositions, such as “on,” “side” (as in “sidewall”),“higher,” “lower,” “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate.

There is a need in the industry, as recognized by the inventors herein,to undercut (100) silicon using wet etch chemistry. A buffered fluorideetch solution may be used to create square corners and lateral shelvesin (100) silicon without the typical bevel experienced with hydroxideetches when the initial pattern is oriented along the <100> direction.The wet etch chemistry of the present invention may be used to fabricatedevices that have previously been prohibitively expensive, complicatedand/or poor yielding.

An embodiment of the present invention further includes methodsemploying etchant solutions to manipulate the cavity shape of a trenchunderlying single crystal silicon. Etch chemistry is highly selective tocrystal orientation, when using (100) crystal plane orientation andpatterning in a <100> direction, a cavity shape lacking beveled cornersand including a lateral shelf may be achieved.

An embodiment of the present invention includes a method of etching the(100) crystal silicon plane 2-3 times slower than the (110) and (111)silicon planes. The etch rate of (100) silicon may be approximately5-10,000 Å/min and preferably 10-500 Å/min in dilute etchants at lowtemperatures. The method may include exposing the silicon to a bufferedfluoride etch solution of the present invention. The method may furtherinclude a simultaneous slower etch on an oxide and/or nitride relativeto the (100) silicon.

In an embodiment of the present invention, a square undercut in singlecrystal silicon may be created by providing single crystal siliconincluding at least one trench therein, patterning the single crystalsilicon in the <100> direction and exposing the single crystal siliconto a solution including a fluoride component, an oxidizing agent and aninorganic acid.

In an embodiment of the present invention, a lateral shelf may becreated by exposing single crystal silicon to an anisotropic etchantfollowed by a buffered fluoride etch solution. Alternatively, a lateralshelf may be created by exposing single crystal silicon to a firstisotropic etchant to create a trench. An anisotropic etchant may beapplied to undercut the silicon and a buffered fluoride etch solutionmay be applied to square the corners of the undercut cavity. It will beunderstood that the buffered fluoride etch solution, which etchessilicon at different rates in different exposed planes, may be used inthe trench without a first anisotropic etchant.

An embodiment of the present invention includes a semiconductor deviceincluding single crystal silicon having a square undercut feature. Theundercut feature includes smooth surfaces. An embodiment of the presentinvention includes a semiconductor device including single crystalsilicon having a lateral shelf.

Etch compositions for oxidizing silicon and etching silicon dioxide tocreate desired structures according to the present invention shall begenerally described below. With the description as provided below, itwill be readily apparent to one skilled in the art that the bufferedfluoride etch compositions described herein may be used in variousapplications. In other words, the buffered fluoride etch compositionsmay be used whenever silicon etch is being performed and wherein squareundercuts or lateral shelves are desired. For example, the presentinvention may be used in the formation of isolation structures for usein the fabrication of integrated circuits. Further, for example, thepresent invention may be beneficial in the fabrication of transistorstructures, such as pseudo-silicon-on-insulator devices (including DRAM,SRAM, Flash, imagers, PCRAM, MRAM, CAM, etc.), FinFets, surround gatetransistors, as well as micro electronic mechanical systems (“MEMS”) andelectro-optical components.

In one embodiment, a buffered fluoride etch composition for use inundercutting single crystal silicon to form lateral shelves generallyincludes a fluoride component, an inorganic acid and an oxidizing agent.The fluoride component may be, without limitation, HF, HF₂—, NH₄F, ortetramethylammonium fluoride (TMAF). The ammonium fluoride may be formedwith a mixture of ammonium hydroxide and HF. The fluoride component orsolution is such that when the reaction of the etch composition withsilicon forms silicon dioxide, the fluoride component or solutiondissolves away the silicon dioxide formed thereby. The fluoridecomponent may be present in the amount of 0.5-50% by weight.

The oxidizing agent of the buffered fluoride etch composition may be anyoxidizing agent such as, for example, hydrogen peroxide or ozone. Onecurrently preferred oxidizing agent is hydrogen peroxide.

The inorganic acid component may include at least one acid selected fromhydrofluoric acid (HF), phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄),nitric acid (HNO₃), hydrochloric acid (HCl), carbonic acid (H₂CO₃), orany other suitable inorganic acid. It is currently preferred that theinorganic acid be H₃PO₄ or H₂CO₃. Inorganic acids are commerciallyavailable as concentrated solutions (X) which then typically are dilutedto a desired concentration (H₂O:X). For example, commercially availableconcentrated acids are available as follows: HCl is 37% by weight indeionized water; HNO₃ is 70% by weight in deionized water; H₂SO₄ is 96%by weight in deionized water, and H₃PO₄ is 85% by weight in deionizedwater. Concentrations of etch compositions described herein are givenbased on commercially available solutions. For example, if the etchcomposition has a concentration of 30% HCl, then the solution includes30% by weight of the commercially available HCl solution. Hydrogenperoxide (H₂O₂) is also commercially available as a concentratedsolution of approximately 29% by weight in deionized water. Further,ammonium fluoride is also commercially available as a concentratedsolution, approximately 40% by weight in deionized water. Further, onewill recognize that multiple components of the solution may be providedfrom commercially available solutions. For example, a wet etch solutionmay be employed that provides both NH₄F (˜39.4 w %) and an inorganicacid (i.e., H₃PO₄˜0.6 w %) that may be used to adjust the pH of thesolution.

Other exemplary suitable etchants are disclosed in U.S. patentapplication Ser. No. 10/625,166 and U.S. Pat. No. 6,391,793 the contentsof each of which document is incorporated herein by reference. Thebuffered fluoride etch solution preferably has a pH in the range ofabout 5.0 to about 9.0. More preferably, the buffered fluoride etchcomposition has a pH of about 7.8. Preferably, the buffered fluorideetch composition includes a fluoride component in a range of about 0.5percent to about 50 percent by weight of the buffered fluoride etchcomposition, an oxidizing agent in the range of about 0.5 percent toabout 30 percent by weight of the buffered fluoride etch composition;and an inorganic acid in the range of about 0.1-2% by weight. Forexample, the buffered fluoride etch composition may preferably include avolumetric ratio of NH₄F:QEII:H₂O₂ of about 4:2:3.

Further, preferably, the ionic strength of the buffered fluoride etchcomposition is greater than one; more preferably, the ionic strength isin the range of about five to about 20. As used herein, ionic strengthrefers to a measure of the average electrostatic interaction among ionsin the composition, which is equal to one-half the sum of the termsobtained by multiplying the molality of each ion by its valence squared.Yet further, preferably, the redox potential of the etch composition isin the range of about −0.5 to about +0.7 or higher (vs. StandardHydrogen Electrode (SHE)). As used herein, the redox potential is ameasure of the effectiveness of the etch composition as an oxidizingagent, i.e., the ability of the etch composition to oxidize silicon forremoval by the HF component of the etch composition.

The above ranges for the buffered fluoride etch solution areparticularly applicable to the use of ammonium fluoride and hydrogenperoxide but appear to be equally applicable to buffered fluoride etchcompositions having other combinations of components as described above,such as when ammonium fluoride is provided by ammonium hydroxide andhydrofluoric acid. In other words, suitable amounts of ammoniumhydroxide and hydrofluoric acid may be mixed to provide an adequateamount of ammonium fluoride. When ozone is used as the oxidizing agent,ozone is preferably present in a range of about 1 part per million (ppm)to about 50 ppm.

The buffered fluoride etch solution may exhibit an etch rate of siliconthat is greater than three times the etch rate of an oxide being exposedto the same etch composition, i.e., the selectivity between silicon andoxide is greater than 3. More preferably, the selectivity betweensilicon and oxide using the etch composition is greater than 6 comparedto <100> silicon etch rate.

Further, to achieve desired throughput of wafers, the etch rate forsilicon using the etch composition is preferably greater than about 5Å/min. More preferably, the etch rate for silicon is greater than 18Å/min. Even more preferably, the etch rate for silicon is greater than30-50 Å per minute.

Preferably, the etch composition is such that after removal of siliconusing the etch composition the silicon surface has a desired surfaceroughness adequate for later processing. Preferably, the roughness ofthe silicon surface following the etch is within the range of about 1.25Å RMS to about 1.30 Å RMS. The silicon surface may desirably fall withinsuch a range for roughness after more than 180 Å of silicon is removed.Generally, for example, roughness may be determined by Atomic ForceMicroscopy (AFM) which scans a surface area of about 1 μm² and gives anaverage peak-to-valley measurement across this 1 μm² surface area, rms.

Preferably, the high selectivity to oxide as described above is a highselectivity to thermal oxide. For example, such thermal oxide may beformed by thermal oxidation such as with use of a wet or dry furnaceoxidation. However, such selectivity is also applicable to oxides formedby chemical vapor deposition (CVD), such as high-density plasma oxidetypically used in isolation processes, such as shallow trench isolation.

Generally, any known method may be used to expose the silicon to thebuffered fluoride etch solution. For example, the silicon may beimmersed into a tank of the buffered fluoride etch solution. Thesolution may also be sprayed onto the wafers being etched or may beintroduced for contact with the wafer in any other manner, e.g., drip,spraying, vapor, etc. The etching process may be performed at atemperature in the range of about 10° C. to about 90° C. Preferably, theetching process occurs at a temperature between 21° C. to about 30° C.and, more preferably, between about 22° C. and 25° C.

As previously described herein, FIGS. 1A-2B show that when NH₄OH or TMAHis used to undercut silicon, virtually no lateral shelf is formed. Whenperformed at a 45° rotation (i.e., patterned along the <100> direction)the corners of the undercut structure have chamfers 12. (FIG. 1B andFIG. 2B). These resulting structures are undesirable for manymanufacturing processes. Referring to FIGS. 3A and 3B, a silicon 10undercut was performed in both the standard silicon orientation (i.e.,patterned along the <110> direction) and 45° rotation in (100) siliconusing a buffered fluoride etch solution (10 L NH₄F+5 L QEII+7.5 L H₂O₂)at 26° C. according to the present invention. The buffered fluoride etchsolution used in FIGS. 3A and 3B demonstrates that the (100) siliconplanes is the slow etching planes which allows the creation of squareundercuts if the pattern is aligned along the <100> direction. In atypical hydroxide-based etch, the (111) plane is a slow etch; thus, itwas surprising to discover a wet etch with a slow plane etch in (100)silicon.

The buffered fluoride etch solution provides very useful selectivity,smooth surfaces and controllable etching of (100) silicon. Referring toFIG. 3B, a lateral shelf 14 and lack of beveled corners enables the easycreation thereon of electrical devices such as FinFETs, Pseudo-SOI orRAD bowls in standard CMOS wafers which are manufactured on (100)silicon. The use of the buffered fluoride etch solution also createsconcave square corners 20 without a lateral spacer, which is desirablefor electronic properties in silicon fingers of sheets which have verydifferent properties from the adjacent material having no materialetched. The concave square corners 20 depicted in FIG. 3B are alsouseful for a discrete change in device mechanical and optical propertieswhen fabricating MEMS. The concave square corners may be formed by afirst trench wall 18 substantially perpendicular to a surface of thesilicon and a second, undercut trench wall 16 substantially parallel tothe surface of the silicon. The concave square corners 20 may define asquare undercut feature, such as lateral shelf 14 that includes at leastone region (e.g., second, undercut trench wall 16) that extends under aportion of the silicon. The concave square corners 20 in (100) siliconalso allow simple integration in CMOS devices and enables MEMSmechanical and optical structures to be integrated with CMOS processingmore easily.

The etch rate and selectivity of the buffered fluoride etch solutiondepends on two competing mechanisms—the oxidation of silicon and theetch rate of oxide. This may be depicted in the following simplifiedreactions:Si+2H₂O₂═H₂SiO₃+H₂O═SiO₂+2H₂O  (1)Half-cell reduction/oxidation reactions:H₂O₂+2H⁺+2e−

2H₂O E^(0′)=+1.77 V  (2)Si_(s)+2OH⁻

=Si(OH)₂+2e ⁻  (3)H₂SiO₃+6HF

H₂SiF₆+3H₂O  (4)The typical selectivity between (100) silicon crystal orientation andthermal oxide is approximately six. The (110) directional etch isapproximately two and one half times higher than (100) silicon etch.

Although the buffered fluoride etch solution may be used in variousapplications, FIGS. 4-11D depict a partial process for creating apseudo-SOI structure according to a method of the present invention. Ineach of FIGS. 4A-11D, part A shows a plan view of the structure and partB shows a cross-sectional view of the corresponding structure takenalong -B-. FIGS. 4A and 4B depict a single crystal silicon substrate100. A silicon nitride liner 112 is formed thereover. A masking layer128, for example, photoresist, is formed over the silicon nitride liner112 as known in the art. The masking layer 128 may be patterned to format least one trench mask opening 132. Conventional photolithography orother lithographic or non-lithographic methods, regardless of thepresence of the masking layer 128, are also contemplated.

Referring to FIGS. 5A and 5B, the silicon nitride liner 112 and singlecrystal silicon substrate 100 are etched through the mask opening 132 toform at least one trench 116 within the single crystal silicon substrate100. The etch may be conducted utilizing a dry anisotropic etchingchemistry, with or without plasma, for example comprising ammonia and atleast one fluorocarbon. Masking layer 128 may remain or may be removedwhen etching into the single crystal silicon substrate 100. While aspecific method of forming trench 116 has been disclosed, it will beunderstood by one of skill in the art that any method of forming trench116 may be utilized.

Referring to FIGS. 6A and 6B, a nitride layer may be deposited over thesilicon nitride liner 112 and trench 116 followed by an etch whichremoves the nitride from the bottom 126 of the trench 116, but createsnitride spacers 118 on the sidewalls 130 of trench 116. Creation of theSi₃N₄ liner may be performed by conventional techniques.

The buffered fluoride etch solution of the present invention may then beapplied to undercut the single crystal silicon substrate 10. Preferably,the buffered fluoride etch solution may be applied at approximately 23°C. for approximately 5 minutes, depending on the desired size of thelateral shelf 114. As shown in FIGS. 7A and 7B, the buffered fluorideetch solution etches faster in a direction parallel to the singlecrystal silicon substrate 100 as compared with the vertical etch throughthe bottom 126 of the trench 116. A lateral shelf 114 having a thicknessof approximately 450 Å to 550 Å may be created as shown in FIG. 7B.

If desired, a nitride liner 120 may be deposited on the bottom 126 andsidewalls 130 of the trench 116 and then the trench 116 may be filledwith an oxide material 122, for example, a spin-on-dielectric (SOD) asshown in FIGS. 8A and 8B.

A mask 124 is deposited and patterned over the silicon nitride liner 112and oxide material 122. A conventional silicon etch having someselectivity to oxide may be performed as shown in FIGS. 9A, 9B and10A-10C.

An optional nitride liner 136 may be deposited and an SOD fill may beperformed as shown in FIGS. 11A-11D. After the SOD fill depicted inFIGS. 11A-11D, the structure 150 may be subjected to further processingto faun, for example, transistors, capacitors and digit lines thereoverto complete the pseudo-SOI structure. The structure 150 includes alateral shelf 114 having a thickness of about 500 Å (+/−10%).

The resulting structure, including any transistors, such as arraytransistors or access transistors, overlying structure 150, hassignificantly lower leakage due to the presence of oxide material 122underlying the silicon. (See, e.g., FIG. 11B). It will be understoodthat structure 150 is not limited to being an intermediate pseudo-SOIstructure. Any number of additional fabrication steps may be performedin conjunction with the present invention to create any desired device.

FIGS. 12A-12E depict silicon oxidation and etching using a solution ofNH₄F, QEII and H₂O₂ (provided in a ratio of 4:2:3). The substrate wasimmersed in a stagnant bath of the NH₄F, QEII and H₂O₂ solution at 23°C. FIG. 12A depicts a trench 310 in single crystal silicon 300 with anitride liner 320 prior to addition of the buffered fluoride etchsolution of the present invention. A top surface 312 of the singlecrystal silicon represents the (100) plane. The trench 310 is <100> onthe (100) plane. After 16 minutes of exposure to the buffered fluorideetch solution at approximately 23° C., an undercut profile is visiblehaving a lateral shelf 314. (FIG. 12B) The etch is progressing fasterperpendicular to the (100) direction (i.e., perpendicular to the STIsidewall), than in the (100) direction (i.e., perpendicular to the wafersurface) as shown in FIGS. 12C, 12D and 12E after 22 min, 25 min and 28min exposure, respectively. As seen in FIGS. 12A-12E, the width of theunderlying silicon leg, or pillar, 350, decreases with increasedexposure to the buffered fluoride etch solution.

The buffered fluoride etch solution may be combined with othercomponents in combination with pattern angles to manufacture verticalwalls in various ways. FIG. 13A-13D depicts the etch progression ofsingle crystal silicon 400 at 0 min (FIG. 13A), 3 min (FIG. 13B), 6 min(FIG. 13C) and 9 min (FIG. 13D) exposure to the NH₄F, QEII and H₂O₂solution (the buffered fluoride etch solution) after a five minuteanisotropic NH₄OH etch at 23° C. Exposure occurred using a stagnantbath. A top surface 412 of the single crystal silicon represents the(100) plane. A trench 410 is <100> on the (100) plane. Increasing thetime of the buffered fluoride etch solution etch forms a shelf undercutof the silicon active area without significantly increasing the trenchdepth. Further, it can be seen that the silicon legs, or pillars, 450under the single crystal silicon 400 becomes increasingly narrow as theetch progresses. Thus, it will be understood that using appropriatepattern angles in combination with etchant solutions of the presentinvention, devices may be manufactured having various characteristics.By manipulating the etch time and etchant combination, differentundercut profiles may be achieved. For example, the buffered fluorideetch solution may be combined with hydroxides, NH₄OH, NH₄F, TMAH orcombinations thereof.

The invention may further be understood by the following non-limitingexamples.

EXAMPLE 1

FIGS. 14A and 14B depict two TEMs of an integrated PSOI DRAM accessstructure. The profile was created by the combination of a TMAH (100:1)etch of 4 minutes 36 seconds at 25° C. A second etch using the bufferedfluoride etch solution (NH₄F, QEII and H₂O₂ provided in a ratio of4:2:3) was run at 25° C. for 6 minutes. A conventional oxide spacer wasused for the two wet etches and removed after the cavity creation. Theimage is shown in the <100> direction after the access transistors andbit line were integrated along with poly-silicon plugs between thetransistor gates.

All documents cited herein are incorporated in their entirety as if eachwere incorporated separately. This invention has been described withreference to illustrative embodiments and is not meant to be construedin a limiting sense. As described previously, one skilled in the artwill recognize that various other illustrative applications may utilizethe etch compositions described herein. Various modifications of theillustrative embodiments, as well as additional embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto this description. While the preferred embodiments of the presentinvention have been described herein, the invention defined by theclaims herein is not limited by particular details set forth in theabove description, as many apparent variations thereof are possiblewithout departing from the spirit or scope thereof.

1. A semiconductor structure, comprising at least one trench formed insingle crystal silicon and including a square undercut feature having atleast one region extending under a portion of the single crystalsilicon; and at least one extension of the single crystal siliconprotruding into the square undercut feature.
 2. The semiconductorstructure of claim 1, further comprising at least one nitride structuredirectly on and in contact with at least a portion of a surface of thesingle crystal silicon and extending into the square undercut feature.3. The semiconductor structure of claim 1, wherein the at least oneextension of the single crystal silicon protrudes into the squareundercut feature in a direction substantially perpendicular to a surfaceof the single crystal silicon.
 4. The semiconductor structure of claim1, wherein the at least one extension of the single crystal siliconprotrudes from a sidewall of the at least one trench.
 5. Thesemiconductor structure of claim 1, wherein the square undercut featurecomprises a first trench wall substantially perpendicular to a surfaceof the single crystal silicon and a second trench wall substantiallyparallel to the surface of the single crystal silicon.
 6. Thesemiconductor structure of claim 5, wherein the at least one extensionof the single crystal silicon extends past the second trench wall. 7.The semiconductor structure of claim 1, wherein the at least one trenchcomprises a substantially diamond-shaped region when aligned along a<110> direction in the single crystal silicon and a substantiallysquare-shaped region when aligned along a <100> direction in the singlecrystal silicon.
 8. A method of creating a square undercut in singlecrystal silicon, the method comprising: removing single crystal siliconin the <100> direction to form at least one trench therein; andcontacting a portion of single crystal silicon within the at least onetrench with a solution formulated to remove material from a surfaceoriented in the (100) plane of the single crystal silicon at a slowerrate than material from surfaces oriented in either the (110) or the(111) planes of the single crystal silicon to form a square undercutfeature having at least one region extending under a portion of thesingle crystal silicon and having at least one extension of the singlecrystal silicon protruding into the square undercut feature.
 9. Themethod of claim 8, wherein contacting a portion of single crystalsilicon within the at least one trench with a solution formulated toremove material from a surface oriented in the (100) plane of the singlecrystal silicon at a slower rate than material from surfaces oriented ineither the (110) or the (111) planes of the single crystal siliconcomprises contacting the portion of single crystal silicon within the atleast one trench with a solution comprising a fluoride component, anoxidizing agent, and an inorganic acid.
 10. The method of claim 8,wherein contacting a portion of single crystal silicon within the atleast one trench with a solution comprises forming the square undercutfeature defined by concave square corners.
 11. The method of claim 10,wherein forming the square undercut feature defined by concave squarecorners comprises forming the square undercut feature around aprotrusion of the single crystal silicon extending therein.
 12. Themethod of claim 8, further comprising forming a structure over anunrecessed portion of an upper surface of the single crystal silicon andsidewalls of the at least one trench while leaving a portion of thesingle crystal silicon exposed within the at least one trench.
 13. Themethod of claim 8, wherein contacting a portion of single crystalsilicon within the at least one trench with a solution comprisescontacting the portion of single crystal silicon within the at least onetrench with a solution comprising ammonia and at least one fluorocarbon.14. The method of claim 8, wherein contacting a portion of singlecrystal silicon within the at least one trench with a solution comprisescontacting the portion of single crystal silicon within the at least onetrench with a buffered fluoride etch solution.
 15. A semiconductorstructure comprising: at least one opening extending into a singlecrystal silicon substrate and including an undercut region defined byconcave square corners within the single crystal silicon substrate; andat least a portion of the single crystal silicon substrate extendinginto the undercut region.
 16. The semiconductor structure of claim 15,wherein the at least one opening is formed in the single crystal siliconsubstrate oriented in the (100) plane.
 17. The semiconductor structureof claim 15, wherein the undercut region comprises a first trench wallsubstantially perpendicular to an upper surface of the single crystalsilicon substrate and a second trench wall substantially parallel to theupper surface of the single crystal silicon substrate.
 18. Thesemiconductor structure of claim 17, wherein the at least a portion ofthe single crystal silicon substrate extending into the undercut regionextends substantially parallel to the first trench wall.
 19. Thesemiconductor structure of claim 15, wherein the undercut region isdefined by at least one surface extending under a portion of the singlecrystal silicon substrate.